The recent growth in the demand for broadband services has resulted in a pressing need for increased capacity on existing communication channels. The increased bandwidth of fiber optic communication networks is still often insufficient to cope with this demand without utilizing the ability of these fibers to carry large numbers of individual communication channels, each identified by the particular wavelength of the light. This technique is known as dense wavelength division multiplexing (DWDM). The disadvantage of this technique is that the increasing density of wavelength channels places increasing demand on network functionality for connecting the individual channels to individual destination points on a dynamic basis, and for the ability to add or drop an individual wavelength channel into or out of the optical signal. Currently these functions are primarily performed by electronic techniques but the demand for increased network speed calls for these functions to be performed in the optical domain.
In optical communications systems, the use of wavelength selective switching for applications of optical cross-connects (OXC) has attracted much interest because of the goal of fully flexible, networks where the paths of each wavelength can be reconfigured to allow arbitrary connection between nodes with the capacity appropriate for that link at a particular point in time. Although this goal is still valid, it is clear that optical networks will evolve to this level of sophistication in a number of stages—and the first stage of the evolution is likely to be that of a reconfigurable add/drop node where a number of channels of an input signal can be dropped and added from the main path, whose number and wavelength can be varied over time—either as the network evolves or dynamically as the traffic demands vary. The second stage requires that the reconfigurable add/drop node be expanded to include an arbitrary number of input ports, and include the ability to switch any wavelength channel from any of the input ports to any of the output ports without cross-talk from channels of the same wavelength on any of the other input ports appearing at an output port.
The operation of optical switches with one input and two output ports (1×2) or vice versa is well known in the field of telecommunications networks and forms a basic building block for more complex systems, for example, a switch with two inputs and two outputs (2×2) known as an optical cross connect (OXC) can be constructed by a cascaded arrangement of six 1×2 optical switches. Similarly a 3×3 OXC can be designed using three 2×2 OXC switches, a 4×4 OXC using six 2×2 OCX switches, and so on.
These systems, however, generally route all the channels contained in the signal appearing on the input ports to one of the output ports, often without the ability to block an input completely. In order to selectively route individual wavelength channels contained in either of the signals appearing at the input ports, complex design architectures are needed to demultiplex the input signals, reroute the individual wavelength channels using a separate cross connect switch for each wavelength, and remultiplex the rerouted channels onto the output port. This process increases dramatically in complexity with the number of wavelength channels contained in the input signals as is seen for example in U.S. Pat. Nos. 6,658,175 and 6,678,473.